1) Field of the Invention
The present invention relates to new inks and ink making processes useful in environments such as electronic displays, especially full color displays. These new inks are especially useful for electronic displays that use reflected light, like electrowetting displays and electrophoretic displays. The pigment particles in the inks are designed to have no or insignificant response to applied electric fields, thus allowing the pigment particles to move with and not independently from the carrier liquid under an applied electrical field. These inks can be used to replace existing types of dye solutions in electronic displays.
2) Description of Related Art
Stable dispersions of pigment particles in oil can be formulated with nonaqueous ionic dispersants which impart charge on the pigment particles by the preferential adsorption of either cations or anions. These dispersions are called electrocratic as well. This charging of pigment particles in oil is necessary in electrophoretic displays since it is the movement of charged particle through the oil that produces the change in optical state.
Stable dispersions of pigments in oil can be formulated with nonionic dispersants which should not impart significant charge on the pigments. However, pigment particles dispersed with nonionic surfactants have been found to be slightly charged. These charges are not screened by ions in the oil and so the pigment particles respond to electric fields.
Charged particles in oil, even just slightly charged, will move when electric fields are applied. They exhibit two types of motion: electrophoresis and field induced chaining (sometimes called dielectrophoresis). Electrophoresis causes the charged particles to move. They can move and stick to electrodes and to viewing surfaces.
Field induced chaining can arise in dispersions of polarizable particles when the surfaces of those particles can touch. The particles are held in a chain by the coupling of their field-induced polarizations. This chaining arises because many pigments are more electrically conductive than oils. This is especially significant for dispersions of carbon blacks which are highly desirable in displays because of their optical density.
Charging of pigment particles in oil is deleterious in electrowetting displays since it is the movement of the liquid that produces the change in optical state and independent motion of the pigment is deleterious. For example, if the pigment particles are charged, then the application of an electric field causes the pigment particles to stick to the electrode and prevent or significantly degrading the change in optical state as the liquid moves away.
The reason that nonaqueous surfactants generate ionic charges is that they contain a moiety to cause the surfactant to adsorb to the pigment particle surface. For nonaqueous dispersions, this is usually highly polar, hence charge generating chemistries.
Electronic displays have circuitry to “address” individual pixels across a surface. These signals change the optical properties of the pixels, thus creating the image. Some displays are emissive—that is, the pixels emit light when addressed. Some displays are passive—that is, the pixels modulate light, not create it.
Liquid crystal displays (LCDs) create an image by modulating light as it passes through a layer of liquid crystals by changing the arrangement of the crystals. An important feature of LCDs to note is that the imaging process is separate from the color producing process. A consequence of the separation of image formation and color production is that both contribute to the loss of light and hence reduce the optical efficiency of the display. LCDs often require bright rear illumination to produce a color image. The bright illumination requires extra power, especially in brightly lit environments.
Electrowetting displays (EWDs) have pixels that include ink to modulate the color and intensity of light coming from a light source in the rear or incident light reflected off the back surface. The electrowetting ink is a dyed oil solution submersed in a clear water phase. The switching comes after a voltage is applied to the pixel and the low dielectric oil phase contracts allowing the higher dielectric water phase to move closer to the electrode. In EWDs, a liquid phase changes position to change optical properties.
Electrophoretic displays (EDPs) use ink that is a dispersion of at least one type of pigment particle in a low dielectric, insulating oil, generally a low molecular weight hydrocarbon, with an oil-soluble component added to control the electric charge on the pigment particle. For example, an arrangement of white particles in oil dyed blue. In a given pixel, an applied electric field can move the particles to the top (viewing) surface creating a white state, and alternatively move the particles to the rear surface creating a blue state from the dye. Thus, the creation of images with these inks follows from the ability to control the position of the charged pigment particles by applying an electric field.
An advantage of pigment-base ink displays over LCDs is that the image modulation and the color production are combined in one layer. The inherent brightness of such a construction enables these EPDs and EWDs to be used in ambient light, the same as printed images can be read from room light to bright sunshine. However, the inks for EPDs and EWDs have a serious disadvantage in that the dyes available are not sufficiently light-stable. That is, the dye fades with exposure to ambient light.
Other known EPDs contain two types of particles instead of particles and a dye. Generally one color of particles is positively charged and the other color negatively charged. These are called dual particle electrophoretic displays and generally contain no dye in the oil. For example, the dispersion contains two sets of particles, say white and black, in a transparent liquid. The ink could have positive white particles of charge −q and negative black particles of charge +q. The dual particle dispersions use two pigments, both of which are more light stabile than dyes, so that these displays can be used in ambient light in the same way printed images can be read. These inks provide stability to light exposure. However, having two different sets of oppositely charged particles is highly complex.
An important characteristic of the pigments used in EPDs is that the charges on the particles be well controlled. After all, it is the ability of an applied electric field to control the position of the pigment particles that enables the individual pixels to be controlled and images to be formed. For dual particle displays, the two colors of pigment particles must have well separated electric charge so the two different colors can be well separated. However there is also a need for uncharged pigment particles which enable other imaging processes. For example, the combination of charged white particles and uncharged pigment particles in an ink for an electrophoretic display could be used to produce a bright white state and a bright color state. Creating dispersions of pigments without significant electric charge in oils is a challenge not successfully addressed in the prior art.
Using pigment particles in displays rather than dyes is preferred for several reasons, two of which are: (1) the light fastness of pigments is superior to dyes and (2) the optical densities of pigment dispersions can be much greater than dye solutions. However, pigments have not been used in EWDs because, when the known surfactants are added to disperse the pigment particles, the particles become electrically charged. The charged pigment particles respond to the electric fields used in EWDs and move. The motion of the particles is not the same as the motion of the liquids. For example, the particles may move towards the electrode and stick. This leaves a residual color on the electrode and substantially degrades the optical performance of the display. Particles can chain in an electric field and prevent the liquid from moving at all.
Methods and materials to disperse pigments in the oils of EWD inks and not have the pigments respond to electric charge are needed.
In general pigments of different colors have different chemistries at their surfaces. This requires different kinds of reactive chemicals to treat the surfaces. For those skilled in the art of dispersions in oils it is also known that for adsorption of chemicals on the surface there must be some strong interactions between the soluble species and the pigment surface. This may vary from pigment chemistry to pigment chemistry and so different soluble chemicals are needed. These are sometimes antagonistic to each other and useful mixtures of pigments with different treatments are not possible.
What is needed are materials and methods, each of which can treat a wide variety of pigment chemistries.
What is needed are materials and methods to treat conductive pigments like carbon black so that electrical effects are eliminated.
What is needed are materials and methods to treat all pigments without creating electric charges on the particles, even in the presence of other chemical components in the oil.
Accordingly, it is an object of the present invention to enable the use of a single surfactant to disperse multiple kinds of pigment particles.
Accordingly, it is an object of the present invention to disperse pigment particles in oils without significant charge.
Accordingly, it is an object of the present invention to provide inks for electronic displays.
Accordingly, it is an object of the present invention to combine the image formation and color production of a display pixel in one layer by using the position of color pigment particles in a carrier liquid to modulate the image.
Accordingly, it is an object of the present invention to provide inks for use in electronic displays with stability to light exposure.